Pyro-methyl phosphonic acid (pyro-MPA) is useful as an intermediate in the manufacture of various organophosphorus compounds which are useful as flame retardants or for other purposes. More particularly, pyro-MPA is a useful precursor in the manufacture of methyl phosphonic acid (MPA), a compound having the formula ##STR1## which is useful as a flame retardant or as a component of flame retardant systems for fabrics. For instance, MPA may be applied to fabrics as a flame retardant finish in the form of an aqueous solution of its ammonium salt in combination with cyanamide.
Various methods have been previously proposed for making pyro-MPA from dimethyl hydrogen phosphite (DMP), but such methods have generally been relatively unsatisfactory, producing the desired products in relatively low degrees of purity or low yields and often requiring cumbersome manufacturing procedures. For instance, Dawson et al, in U.S. Pat. No. 2,951,863 refer to the slow rate at which DMP, when heated, is converted to a mixture comprising the methyl ester of MPA and pyro-MPA and describe the difficulties encountered when attempts were made to increase the conversion rate by conducting the reaction in various high boiling heat transfer media. More particularly, Dawson et al, report that no conversion whatever of DMP was obtained after three hours of heating when diphenyl ether was used as a heat transfer medium, and that a hard, glassy, brown solid of no apparent use was obtained as the chief reaction product when an attempt was made to pyrolyze DMP using benzophenone as a heat transfer medium. According to Dawson et al, improved results are obtained if a heavy paraffin oil is used as the heat transfer medium but even in such a system the product of the pyrolysis is reported to contain more than 10% methyl ester of MPA and less than 90% pyro-MPA. As the methyl ester is known to be relatively difficult to hydrolyze in the absence of a strong acid and the use of a high boiling reaction medium necessarily further complicates product recovery, the process described by Dawson et al, still leaves much to be desired as a route for the production of substantially pure MPA.
In U.S. Pat. No. 2,923,729, Hardy describes another process for producing a mixture of methyl ester of MPA and pyro-MPA by the pyrolysis of DMP. This process does not rely on the addition of any extraneous solvent, but requires the continuous circulation of a liquid stream between a primary and a secondary reaction zone such that DMP is fed into and partially pyrolyzed in the circulating liquid in the primary zone and its pyrolysis is "completed" in the secondary zone. Thereupon a portion of the pyrolyzed mixture is recovered from the secondary zone as product and another portion returned to the primary zone. According to this procedure, the heating of the liquid mixture in the secondary zone is continued until the DMP has been substantially completely converted to other compounds, principally methyl ester of MPA and pyro-MPA, i.e., until DMP constitutes less than 2% and preferably less than 0.2% of the liquid pyrolysis product. However, as the pyrolysis in such a case is continued only until the DMP concentration in the reaction mixture approaches or reaches zero, it is evident that the product from the Hardy process is essentially the same kind of mixture as in the Dawson et al, process described above, i.e., a mixture comprising a major proportion of pyro-MPA, a minor but substantial proportion of methyl ester of MPA and usually at least a trace of DMP. As in the case of the Dawson et al process, the product is not one that lends itself to be easily hydrolyzed to form MPA nor one from which pure MPA can be readily recovered in high yields.
Other patent references representative of the prior art relating to the pyrolysis of DMP or similar phosphorus compounds and the production of various phosphonic and phosphinic acids or their anhydrides include the following: U.S. Pat. Nos. 2,268,157 Marvel; 2,365,466 Hamilton; 2,397,422 Kosolapoff; 2,559,754 Bittles, Jr., et al; 2,587,340 Berkeley et al; 2,717,906 Lecher et al; 2,853,515 Coates et al; 2,863,900 Beach et al; 2,929,843 Dawson et al; 3,054,821 Rolih et al; 3,089,889 Cleveland et al; 3,093,673 Beach et al; 3,179,695 Weilmuenster et al; 3,708,535 Firestone; British Pat. No. 734,187 (published July 27, 1955); German DOS No. 2,007,784 (published Oct. 15, 1970).
References in the general chemical literature which have been considered include "Synthesis and Properties of Phosphinic and Phosphonic Acid Anhydrides" by Moedritzer, J. Am. Chem. Soc., Vol. 83, p. 4381 (1961); "Addition Reactions of the Phosphorus Halides V. The Formation of an Unsaturated Phosphonic Acid" by Conant and Coyne, J. Am. Chem. Soc., Vol. 44, p. 2530 (1922); "Trichloromethylphosphonic Acid" by Bengelsdorf and Barron, J. Am. Chem. Soc., Vol. 71, p. 2869 (1955); Chemical Abstracts 2685-2687, Vol. 47 (Yakubovich et al); Bull. Soc. Chim., France (1961), 1084 (Monard and Quinchon) DMP Used as Route to MeP(O)Cl.sub.2 By Treatment of the Pyrolyzed Product With SOCl.sub.2 or PCl.sub.5 ; and Memorial des Poudres, 44, 119-32 (1962) (Quinchon et al), Action of SOCl.sub.2 on Products Produced by the Pyrolysis of Dimethyl Phosphite.